1. Field of the Invention
This invention relates to a toner used for electrophotography, electrostatic recording, and the like, and more particularly, to an insulating magnetic toner.
2. Description of the Prior Art
There are known many electrophotographic processes such as those disclosed in U.S. Pat. No. 2,297,691, British Pat. Nos. 1,165,406 and 1,164,405. Most widely used are processes comprising utilizing a photoconductive material, forming electric latent image on a photosensitive material by an optional means, developing the latent images with a toner, if desired, transferring the images thus developed to an image receiving member such as a paper, and then fixing the developed toner images by heat, pressure, solvent vapor or the like. There are known various methods for visualizing electric latent images by a toner. For example, there are known magnetic brush methods as disclosed in U.S. Pat. No. 2,874,063, cascade developing methods as disclosed in U.S. Pat. No. 2,618,552, powder cloud methods as disclosed in U.S. No. 2,221,776, fur brush methods, liquid developing methods and the like.
Among these developing methods, there are widely used, in practice, magnetic brush methods, cascade methods, liquid developing methods and the like where the developer is mainly composed of toner and a carrier. These developing methods can produce relatively stably a good image, but suffer from degradation of carrier and variation of the mixing ratio of the toner and the carrier which are common and inherent drawbacks of two-component developers.
For the purpose of avoiding such drawbacks, it has been proposed to use one-component developers composed of toner only, and among them, methods using a developer composed of magnetic toner particles give a good result.
U.S. Pat. No. 3,909,258 disclosed a process for developing with a magnetically attractable, electronically conductive toner where a developer composed of the toner is carried on a conductive sleeve of drum type having magnets inside and the development is carried out by contacting the developer with electrostatic images. At the developing portion an electrically conductive path is formed by the toner particle between the surface of an image receiving member and the sleeve surface, and electric charge is led from the sleeve to the toner particles through the electronically conductive path, and the toner particles attach to the image portions by Coulomb force to develop the image portions.
The above mentioned development method using the magnetically attractable, electronically conductive toner is a good method free from inherent problems of twocomponent developing methods, but it is difficult to transfer electrostatically the developed images to a final support such as plain paper from the development image bearing member because the toner is electrically conductive.
As a developing method where a highly resistive and magnetic toner capable of being transferred electrostatically is employed, Japanese Patent Laid-Open No. 94140/1977 discloses a process for development utilizing induction polarization of toner particles, but the process suffers from disadvantages such as low development speed and insufficient density of the development images, and is practiced with difficulty.
A further method of development using a highly resistive and magnetic toner is a method comprising triboelectrically charging the toner particles by the friction of toner particles contacting each other, the friction between toner particles and a sleeve, and the like, and bringing the toner particles thus charged into contact with an electrostatic image bearing member to develop the electrostatic images. However, this method suffers from the disadvantages that the amount of contact between the toner particles and the friction member is too small to be sufficiently charged and the toner particles thus charged are more strongly affected by Coulomb force between the toner particles and the sleeve and thereby are liable to agglomerate on the sleeve. The practical operation is effected with difficulty.
Meanwhile, U.S. Pat. No. 4,292,387 proposed a novel development method overcoming the above-mentioned disadvantages. In this method, an extremely thin layer of a magnetic toner is formed on a sleeve, charged triboelectrically, and in a magnetic field, is brought very closely to an electrostatic image to face each other without contact, thus developing the image. According to this method, the extremely thin layer of toner increases the opportunity of toner-sleeve contact, thereby permitting generation of a sufficient amount of triboelectricity; the toner is held on the sleeve by the action of magnets contained therein and is moved in relation to the magnets, thereby rubbing sufficiently the toner with the sleeve as well as deagglomerating aggregates of toner particles; and background fogging is prevented by developing the electrostatic image with the toner which is held by the magnetic force and opposed to the image without contact; whereby an excellent image can be obtained. However, since the insulating toner used in this development method contains a considerable amount of a finely divided magnetic material dispersed in a binder resin, and a part of the magnetic material is found on the surface of the toner particles, the dispersion degree of the magnetic material in the binder resin has a great influence on the free flow and triboelectric property of the magnetic toner, thus affecting the variation and deterioration of development characteristics, durablity, and other properties of the toner. The dispersion degree of the magnetic material in the binder resin in the mixing and kneading steps of the toner production process intimately relates to the dispersion degree in the particles of the final product toner, thus affecting performance characteristics of the toner to a large extent. Moreover, under high humidity service conditions or other unfavorable conditions, the toner exhibits poor free flow and hence tends to form aggregates, which can not be throughly deagglomerated by magnetic force, and the triboelectric charging of the toner becomes insufficient, thus deteriorating the quality and density of image. As stated above this improved development method involves unstable factors concerning characteristics of the magnetic material and is liable to be affected by environmental conditions.
Known magnetic powders used for magnetic toners are ferromagnetic elements and alloys and compounds thereof, including those containing iron, cobalt, nickel, manganese, or zinc such as magnetite, maghemite, ferrite, and the like. Properties known to be required for magnetic powders of these materials are, for example, (1) maximum magnetizing force .sigma.m of at least about 40 emu/g, (2) coercive force Hc of about 150-500 Oe, (3) electric resistivity of 10.sup.2 -10.sup.7 .OMEGA..multidot.cm, (4) sufficient blackness for practical use, (5) good humidity resistance, and (6) good miscibility for resin. Magnetic toners are made in many cases from magnetite which is widely used as a pigment called "iron black" and magnetite is after mentioned in various patents of the prior art. Magnetite almost fulfills the above-mentioned requirements, but in order to use it for an insulating magnetic toner, sufficient examinations are necessary on the miscibility of magnetite with resin and the toner agglomeration tendency, triboelectric chargeability, and durability.